Quick Tech With Mar­lan Davis

Hot Rod - - Contents -

Ef­fec­tive in-ve­hi­cle torque con­verter stall speed is de­fined as the rpm the en­gine can reach with the brakes locked and the trans­mis­sion in gear be­fore the drive wheels turn. Con­verter stall speed must be high enough to put the en­gine into the torque range where it can most ef­fi­ciently launch the car. If con­verter stall speed is too low, the car will be lazy leav­ing the line; if too high, there’ll be ex­ces­sive high-gear slip­page—ei­ther case adds time to your e.t.

En­gine builders use car weight and dyno data to aid se­lec­tion. The con­verter is also matched to the trans­mis­sion’s gear ra­tios. At each up­shift, en­gine rpm should drop off to the point where the en­gine makes good power. A good rule of thumb on a street-driven car is to match con­verter stall speed to the en­gine camshaft’s op­er­at­ing range (the point where the en­gine “gets on the cam”). Rac­ers should se­lect stall speeds close to the en­gine torque peak.

Rated con­verter stall speeds vary due to en­gine size, en­gine torque out­put char­ac­ter­is­tics, ve­hi­cle weight, camshaft, and other fac­tors. A large cu­bic-inch en­gine will raise a given con­verter’s stall speed. En­gines that pro­duce more low-end torque cause a given con­verter to stall at a higher rpm. A heavy car with large di­am­e­ter tires will raise ef­fec­tive stall speed as in­stalled in the ve­hi­cle. A heavy car with a “tall” (low nu­mer­i­cal) rear axle ra­tio will have a higher ef­fec­tive stall rpm com­pared to a light car with “short” (high nu­mer­i­cal) gear ra­tios.

TCI [ A typ­i­cal TCI 10-inch StreetFighter con­verter is de­signed to work best with 280- to 300-de­gree ad­ver­tised du­ra­tion cams, 3.55–4.56:1 rear gears, and higher than stock com­pres­sion ra­tios. Rated stall is 3,000–3,400 rpm in a typ­i­cal small-block, but 3,400–3,600 rpm in a big-block. PN 142200, shown here, fits 1967–1982 727 TorqueFlites with a 24-spline in­put shaft.

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